Mountain's skull

The Mountain's skull is an enormous human skull reputed to be the skull of the deceased Ser Gregor Clegane.[1] The skull's brow shelf is thick and heavy, the is jaw massive. It looks as if it is grinning.[1]

The feast commences and continues late into the night, presided over by the grinning skull on its pillar of black marble. Spun-sugar skulls are served, which displeases Balon. After the feast is done Maester Caleotte hurries behind Doran's wheelchair cradling the Mountain's skull as it were a child.[1]

In Doran's solar Obara wonders why the skull was stripped to the bone. Doran notes that the skull is large and Tyene confirms that if the poison used by her father, Oberyn, broke the Mountain's skin, then Gregor is dead. Obara gives the skull a mocking kiss.[1]

Quotes

Qyburn: I am sorry that it took so long. Such a large head. It took the beetles many hours to clean the flesh. By way of pardon, I have lined a box of ebony and silver with felt, to make a fitting presentation for the skull.Cersei: A cloth sack would serve as well. Prince Doran wants his head. He won't give a fig what sort of box it comes in.[4]

No one saw the Mountain die, and no one saw his head removed. That troubles me, I confess, but what could the bitch queen hope to accomplish by deceiving us? If Gregor Clegane is alive, soon or late the truth will out. The man was eight feet tall, there is not another like him in Westeros. If any such appears again Cersei Lannister will be exposed as a liar before all the Seven Kingdoms. She would be an utter fool to risk that. What could she hope to gain?[1]